1. Field of the Invention
The present invention relates to battery fuel gauges for indicating the state of charge or discharge, typically of a rechargeable battery.
2. Prior Art
As used herein, the word battery will be used generally, and is to be understood to include multiple batteries and battery packs, that is, multiple batteries packaged and electrically connected to functionally appear as a single battery of a higher voltage and/or higher current capacity.
In many rechargeable battery powered devices, it is desirable to know the state of charge (or discharge) of the battery during its discharge cycle so that the user of the battery powered device can appropriately time the recharging of the battery, and when necessary, limit the use of the battery powered device to more essential uses to preserve power before the next recharging of the battery. One way this is done in the prior art is to use battery fuel gauges, generally referred to as coulomb counters, which monitor the current provided to and taken from the battery, which for a battery of a given amp-hour rating, can be used to predict what fraction of the amp-hour rating remains in the battery throughout its discharge cycle. Such devices are readily commercially available, such as by way of example, the DS2745, the DS2782 and DS2780 fuel gauges manufactured and sold by Maxim Integrated Products of Sunnyvale, Calif., assignee of the present invention.
Such devices are useful for the intended purpose, though have certain characteristics that are less than ideal. By way of example, since such devices monitor current to and from the battery, a current sensor is required, normally in the form of a current sense resistor in series with the battery, so that the voltage drop across the resistor, positive or negative, can be monitored. Since battery fuel gauges typically are designed to operate on relatively low sense voltages, the voltage drop across the current sense resistor in series with the battery need not be large, though still that voltage drop represents a loss of battery voltage and power available to the circuitry powered by the battery, as well as power dissipation (heating) in the resistor itself. Normally, such resistors are discrete components, a disadvantage itself, particularly in battery powered devices such as laptop computers and cell phones where circuit area and cost are to be held to a minimum.
Another undesirable characteristic of such battery fuel gauges is the fact that they essentially operate as integrators, and as such, their output at any time is subject to an accumulation of integration errors since the last time they were set to a reference. Accordingly, such battery fuel gauges work satisfactorily when a battery is regularly fully charged, or alternatively, fully discharged, to provide the reference, but do not perform well if the battery is repeatedly only partially charged and partially discharged. Additionally, a coulomb-counter fuel-gauge can perform no better than its periodic voltage-based corrections, which often occur near full or empty.
Another approach to obtaining an indication of the state of charge of rechargeable batteries is to simply monitor the terminal voltage of the battery and to provide a warning to the user when the terminal voltage begins to approach the voltage at which the battery powered device will automatically shut down. This, of course, has the advantage of simplicity and avoidance of the requirement of a current sense resistor in series with the battery, though has the disadvantage of not providing an ongoing indication of the state of charge of the battery because of not considering the load on the battery at the time of the indication. In that regard, many battery operated devices do not have a constant load, but rather a variable load, depending on what capabilities of the battery powered device are in use at the particular time. By way of example, in cell phones, the battery power required for transmitting is far greater than the battery power required to listen for a new incoming call. Accordingly, a battery voltage may be adequate to detect an incoming call, though may drop too much when trying to place or respond to a call because of the higher current demand on the battery for such functions.